(1) Field of the Invention
This invention is related to microelectronics and, more particularly, to a method of forming very low resistance ohmic contacts on p-type InP.
(2) Description of the Prior Art
Recently InP has attracted much attention because of its wide use in the fabrication of laser diodes and detectors operating in the 1.3-1.7 .mu.m (1 .mu.m=10.sup.-6 meters) wavelength region. Specific contact resistances in the low 10.sup.-6 ohm-cm.sup.2 have been reported by Kuphal (Solid Electron: Vol. 24, p. 69, 1981); and Morkoc et al, IEEE Trans. Electron devices ED-28-1(1981) which are incorporated herein by reference. However, much progress remains to be made in the case ohmic contact on p-type InP. Kuphal and Ericson et al have reported the lowest specific contact resistances, approximately 1.1.times.10.sup.-4 ohm-cm.sup.2 and 10.sup.-4 ohm-cm.sup.2 on p-type InP formed by evaporation of Au-Zn and Au-Mg respectively. Due to the high vapor pressure of zinc (Zn) and magnesium (Mg) at the evaporation temperatures, it is very difficult to control the thickness of the evaporated zinc or magnesium, particularly in the 50-nm (1 nm=10.sup.-9 meters) range required. It has been found that even when an Au-Zn or Au-Mg alloy is used, the Zn and Mg evaporate first. Sputtering of the alloys is an alternative, but it is not compatible with lift-off patterning technique due to the good step coverage achieved in sputtering. Added to this difficulty is the severe contamination of the vacuum system used by zinc evaporation or sputtering. Thus, it is essential to have an easy and well-controlled process to plate on p-type InP which leads to a relatively low resistance ohmic contact. Additional requirements on a desirable process are that (1) small geometry contacts can be made easily through conventional photoresist masking techniques on the p-type areas of the sample and (2) these contacts can be sintered simultaneously with Ni/Ge/Au contacts formed on the n-type areas of the sample.